System and method for a MEMS transducer

ABSTRACT

According to an embodiment, a microelectromechanical systems (MEMS) transducer includes a substrate with a first cavity that passes through the substrate from a backside of the substrate. The MEMS transducer also includes a perforated first electrode plate overlying the first cavity on a topside of the substrate, a second electrode plate overlying the first cavity on the topside of the substrate and spaced apart from the perforated first electrode plate by a spacing region, and a gas sensitive material in the spacing region between the perforated first electrode plate and the second electrode plate. The gas sensitive material has an electrical property that is dependent on a concentration of a target gas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/749,102, filed on Jun. 24, 2015, now U.S. Pat. No. 9,658,179 andentitled “System and Method for a MEMS Transducer,” which application ishereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to microfabricated structures,and, in particular embodiments, to a system and method for amicroelectromechanical systems (MEMS) transducer.

BACKGROUND

Transducers convert signals from one domain to another and are oftenused as sensors. One common transducer used as a sensor and seen ineveryday life is a microphone, which converts sound waves to electricalsignals. Another example of a common sensor is a thermometer. Varioustransducers exist that serve as thermometers by transducing temperaturesignals into electrical signals.

Microelectromechanical systems (MEMS) based sensors include a family oftransducers produced using micromachining techniques. MEMS, such as aMEMS microphone, gather information from the environment by measuringthe change of physical state in the transducer and transferring atransduced signal to processing electronics that are connected to theMEMS sensor. MEMS devices may be manufactured using micromachiningfabrication techniques similar to those used for integrated circuits.

MEMS devices may be designed to function as, for example, oscillators,resonators, accelerometers, gyroscopes, thermometers, pressure sensors,microphones, microspeakers, and micro-mirrors. As an example, many MEMSdevices use capacitive sensing techniques for transducing the physicalphenomenon into electrical signals. In such applications, thecapacitance change in the sensor is converted into a voltage signalusing interface circuits.

One such capacitive sensing device is a MEMS microphone. A MEMSmicrophone generally has a deflectable membrane separated by a smalldistance from a rigid backplate. In response to a sound pressure waveincident on the membrane, it deflects towards or away from thebackplate, thereby changing the separation distance between the membraneand backplate. Generally, the membrane and backplate are made out ofconductive materials and form “plates” of a capacitor. Thus, as thedistance separating the membrane and backplate changes in response tothe incident sound wave, the capacitance changes between the “plate” andan electrical signal is generated.

MEMS, such as capacitive MEMS for example, are often used in mobileelectronics, such as tablet computers or mobile phones. In someapplications, it may be desirable to provide sensors with new orincreased functionality in order to provide additional or improvedfunctionality to the electronic system, such as a tablet computer ormobile phone, for example.

SUMMARY

According to an embodiment, a microelectromechanical systems (MEMS)transducer includes a substrate with a first cavity that passes throughthe substrate from a backside of the substrate. The MEMS transducer alsoincludes a perforated first electrode plate overlying the first cavityon a topside of the substrate, a second electrode plate overlying thefirst cavity on the topside of the substrate and spaced apart from theperforated first electrode plate by a spacing region, and a gassensitive material in the spacing region between the perforated firstelectrode plate and the second electrode plate. The gas sensitivematerial has an electrical property that is dependent on a concentrationof a target gas.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a system block diagram of embodiment MEMS transducersystem;

FIGS. 2a and 2b illustrate schematic cross-sectional views of embodimentMEMS gas sensors;

FIGS. 3a and 3b illustrate a top view and cross-sectional view of anembodiment integrated MEMS transducer;

FIGS. 4a and 4b illustrate a top view and cross-sectional view ofanother embodiment integrated MEMS transducer;

FIGS. 5a and 5b illustrate a top view and cross-sectional view of afurther embodiment integrated MEMS transducer;

FIGS. 6a and 6b illustrate a top view and cross-sectional view ofanother embodiment MEMS gas sensor;

FIG. 7 illustrates a block diagram of an embodiment fabrication processfor forming a MEMS transducer; and

FIG. 8 illustrates a block diagram of another embodiment method offorming a MEMS transducer.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of various embodiments are discussed in detailbelow. It should be appreciated, however, that the various embodimentsdescribed herein are applicable in a wide variety of specific contexts.The specific embodiments discussed are merely illustrative of specificways to make and use various embodiments, and should not be construed ina limited scope.

Description is made with respect to various embodiments in a specificcontext, namely gas transducers, and more particularly, MEMS humiditysensors. Some of the various embodiments described herein include MEMStransducers, MEMS microphones, MEMS gas sensors, MEMS humidity sensors,integrated MEMS microphone and humidity sensing systems, interfacecircuits for MEMS transducer systems, and integrated multiple transducersystems. In other embodiments, aspects may also be applied to otherapplications involving any type of sensor or transducer according to anyfashion as known in the art.

As the number of connected devices in daily use has increased throughrapid miniaturization in technology, advancements in mobileconnectivity, and improvements in energy management and efficiency, thedemand for additional functionality has also increased. One area ofadditional functionality is provided through the integration ofadditional sensors. Mobile devices, or devices in general, may includemultiple sensors such as a microphone, an image sensor, anaccelerometer, and a gyroscope, for example. Adding additional sensortypes may be advantageous in some applications. However, in variousapplications, such as mobile devices for example, the space availablefor additional sensors may be limited by both the physical size of thedevice and the added cost of the additional sensors. Thus, theintegration of additional sensors presents opportunities for innovation.

According to various embodiments described herein, a capacitive MEMS gassensor is provided that is suitable for integration. A process forforming, for example, a MEMS microphone includes forming a perforatedbackplate and a deflectable membrane, spaced apart from the perforatedbackplate, over a cavity in a substrate. According to variousembodiments, a similar process for forming a MEMS gas sensor with asimilar structure includes forming a perforated first electrode and asecond electrode, spaced apart from the perforated electrode, over acavity in a substrate. In such embodiments, the spacing between thefirst electrode and the second electrode is filled with a gas sensitivedielectric material. In particular embodiments, the dielectric materialis sensitive to humidity and a capacitive MEMS humidity sensor isproduced. In various embodiments, the capacitive MEMS humidity sensor isintegrated in a same substrate as a MEMS microphone. The layers of thebackplate and membrane for the microphone may be formed simultaneouslywith the first electrode and the second electrode for the humiditysensor. Thus, various embodiments include an integrated capacitive MEMSgas sensor, e.g., a humidity sensor, which may increase thefunctionality of various devices.

FIG. 1 illustrates a system block diagram of embodiment MEMS transducersystem 100 including MEMS transducer 102, application specificintegrated circuit (ASIC) 104, and processor 106. According to variousembodiments, MEMS transducer 102 includes a gas sensor that transducesphysical signals from gases in contact with MEMS transducer 102 intoelectrical signals that may be readout by ASIC 104 and processed byprocessor 106. In specific embodiments, the gas sensor of MEMStransducer 102 includes a humidity sensor that generates an electricalsignal related to the humidity of the ambient environment of MEMStransducer 102.

According to various embodiments, MEMS transducer 102 may includemultiple sensors integrated on a same semiconductor die. Specifically,MEMS transducer 102 includes an acoustic transducer, i.e., a microphoneor a microspeaker, and a gas sensor in some embodiments, and may alsoinclude a temperature sensor in further embodiments (as shown). In someembodiments, the MEMS transducer 102 includes a capacitive MEMS humiditysensor and a capacitive MEMS microphone, both including first and secondsensing electrodes that are formed of the same respective first andsecond layers in the semiconductor structure. Thus, in particularembodiments, the integration leads to small device size and simplefabrication.

In various embodiments, MEMS transducer 102 is coupled to the ambientenvironment through environmental coupling 108. For example, an openingor port, such as an acoustic port, may be provided in a device package(not shown) that includes MEMS transducer system 100. In suchembodiments, the port provides environmental coupling 108 to the ambientenvironment of the device package including MEMS transducer system 100.

According to various embodiments, ASIC 104 includes an amplifier andbias circuit for interfacing MEMS transducer 102. ASIC 104 may alsoinclude an analog to digital converter (ADC) in some embodiments. Invarious embodiments, ASIC 104 may further include additional circuits toperform various additional functions related to interfacing, biasing, orprocessing electrical signals, for example. Further, MEMS transducer 102includes an actuator, such as a microspeaker in some embodiments, andASIC 104 includes driver circuits to driving the actuator. In someembodiments, ASIC 104 is formed on a same semiconductor die as MEMStransducer 102. In other embodiments, ASIC 104 is formed on a separatesemiconductor die as MEMS transducer 102 and packaged together with MEMStransducer 102. Alternatively, ASIC 104 and MEMS transducer 102 may beformed on separate semiconductor dies and packaged in separate packages.

In various embodiments, processor 106 receives electronic signals fromASIC 104 based on transduced signals from MEMS transducer 102. Invarious embodiments, the electronic signals received at processor 106from ASIC 104 are digital signals. In alternative embodiments, theelectronic signals are analog signals. Processor 106 may be a specificapplication processor, a general microprocessor, a field programmablegate array (FPGA), or a central processing unit (CPU). In alternativeembodiments, processor 106 is formed of discrete digital logiccomponents. Processor 106 is attached to a same printed circuit board(PCB) as the package including ASIC 104 and MEMS transducer 102 invarious embodiments.

FIGS. 2a and 2b illustrate schematic cross-sectional views of embodimentMEMS gas sensors 110 and 111. MEMS gas sensor 110 includes substrate112, first electrode 116, second electrode 120, and gas sensitivematerial 122 overlying cavity 113. According to various embodiments, gassensitive material 122 is a dielectric that is sensitive to humidity. Insuch embodiments, as the humidity increases or decreases, the dielectricconstant of gas sensitive material 122 changes accordingly. Based on thechanges in dielectric constant, the parallel plate capacitance formedbetween first electrode 116 and second electrode 120 also changes. Suchchanges in capacitance may be measured by an interface circuit, such asASIC 104, that is coupled to first electrode 116 and second electrode120 through metallization layers (not shown).

In various embodiments, gas sensitive material 122 is in contact withthe ambient environment of MEMS gas sensor 110. Because first electrode116 and second electrode 120 encase gas sensitive material 122 fromabove and below, perforations 134 in first electrode 116 or perforations136 in second electrode 120 provide openings that expose gas sensitivematerial 122 to the ambient environment. In such embodiments,perforations 134 and perforations 136 provide openings to receiveenvironmental signals through environmental coupling 128 orenvironmental coupling 130, respectively. For example, environmentalcoupling 128 and environmental coupling 130 may be ports formed in thepackage that includes MEMS gas sensor 110.

In various embodiments, only one of environmental coupling 128, andcorresponding perforations 134, or environmental coupling 130, andcorresponding perforations 136, is included. In such embodiments,environmental coupling 130, and corresponding perforations 136, maycorrespond to a top port MEMS package. Similarly, environmental coupling128, and corresponding perforations 134, may correspond to a bottom portMEMS package. In such embodiments, cavity 113 in substrate 112 iscoupled to a bottom port in the MEMS package. In alternativeembodiments, both environmental coupling 128, and correspondingperforations 134, and environmental coupling 130, and correspondingperforations 136, are included.

In various embodiments, structural layer 114 is formed on a top surfaceof substrate 112 and separates first electrode 116 from substrate 112.Further, structural layer 118 is formed on top of first electrode 116and separates second electrode 120 from first electrode 116. In variousembodiments, gas sensitive material 122 is structured above cavity 113between first electrode 116 and second electrode 120 while structurallayer 118 is structured above substrate 112 between first electrode 116and second electrode 120. In alternative embodiments, structural layer118 may be removed and gas sensitive material 122 may extend abovesubstrate 112 between first electrode 116 and second electrode 120.

Gas sensitive material 122 is selected based on sensitivity to a certaingas concentration. Specifically, gas sensitive material 122 may beselected based on sensitivity to humidity. In various embodiments, gassensitive material 122 may be selected for use in a resistance orcapacitance based sensor and, therefore, may include materials that varyresistance or dielectric values based on the intended gas concentration.Some resistive materials may require operation at a higher temperature,i.e., may require a heater, in order to be sensitive to gasconcentrations. In some embodiments, some dielectric gas sensitivematerials may have a dielectric constant that is sensitive orproportional to a concentration of a specific gas without requiringheating to higher temperatures. In some embodiments, gas sensitivematerial 122 may be a polymer based material that varies capacitance,i.e., dielectric constant, based on humidity such as polyimide,polyesters, polymethyl methacrylate (PMMA), benzocyclobutene (BCB),polysulfates, and cellulose acetate butyrate. In other embodiments, gassensitive material 122 may be a metal or semiconductor based materialthat varies capacitance, i.e., dielectric constant, based on humiditysuch as aluminum oxide, titanium oxide, silicon oxide, silicon carbide,or porous silicon. In other embodiments, gas sensitive material 122 maybe a material that varies resistance based on humidity such as aluminumoxide, titanium oxide, or various spinels, e.g., BaTiO₃, ZnCr₂O₄,K₂CrO₄, and MgAl₂O₄.

In various other embodiments, gas sensitive material 122 may be selectedto be sensitive to other gas concentrations. In specific embodiments,gas sensitive material 122 may be a material that varies capacitance,i.e., dielectric constant, based on gas concentration. In suchembodiments, gas sensitive material 122 may be a ferroelectric materialfor sensing hydrogen; gas sensitive material 122 may be a fluoropolymer,CuO mixed with BaSnO₃, SrTiO₃, CaTiO₃, ZnO, or BaTiO₃ for sensing carbondioxide; gas sensitive material 122 may be zirconium oxide or an iridiumoxide for sensing oxygen; gas sensitive material 122 may be a tindioxide for sensing ethylene; or gas sensitive material 122 may beporous silicon carbide or titanium oxide for sensing ammonia.

In still further specific embodiments, gas sensitive material 122 may bea material that varies resistance based on gas concentration. In suchembodiments, gas sensitive material 122 may be a palladium basedmaterial for sensing hydrogen; gas sensitive material 122 may be In₂Te₃for sensing carbon dioxide; gas sensitive material 122 may be zirconiumoxide or an iridium oxide for sensing oxygen; gas sensitive material 122may be a tin dioxide for sensing ethylene; or gas sensitive material 122may be tin dioxide, tungsten oxide, various metal oxides, polypyrrole,or polyaniline for sensing ammonia.

In particular embodiments, it may be advantageous for gas sensitivematerial 122 to be formed of a material that is capable of beingdispensed as a liquid or gel. In such embodiments, gas sensitivematerial may be applied post fabrication to a sensing region with twoelectrodes in order to form the MEMS gas sensor. In various otherembodiments, gas sensitive material 122 may be formed of nanostructures,such as nanowires, for example.

According to some specific alternative embodiments, gas sensitivematerial 122 may be simply air. In such embodiments, the dielectricconstant of air is dependent on the humidity, i.e., the amount ofmoisture in the air. Thus, the capacitance may be measured between firstelectrode 116 and second electrode 120 in order to determine changes inthe dielectric constant of the air and, thereby, determine the changesin humidity.

According to various embodiments, substrate 112 may be a single elementsemiconductor substrate, such as silicon, germanium, carbon, orantimony. In other embodiments, substrate 112 may be asilicon-on-insulator (SOI) substrate, a III-V semiconductor substrate,or a II-VI semiconductor substrate. In some embodiments, substrate 112may be glass. In alternative embodiments, substrate 112 may be a polymersubstrate. In other alternative embodiments, substrate 112 may be ametal substrate.

According to various embodiments, first electrode 116 and secondelectrode 120 are formed of a conductive material. In specificembodiments, first electrode 116 and second electrode 120 arepolysilicon. In some embodiments, first electrode 116 and secondelectrode 120 are aluminum, gold, or platinum. In one alternativeembodiment, first electrode 116 and second electrode 120 are copper. Instill another embodiment, first electrode 116 is silicon-on-insulator(SOI). In other embodiments, first electrode 116 and second electrode120 may be formed of a doped semiconductor material. First electrode 116and second electrode 120 may also be formed of a material stackincluding insulating layers that surround and encapsulate a conductivematerial. For example, a polysilicon layer may be encapsulated bysilicon nitride to form first electrode 116 or second electrode 120.

In various embodiments, structural layer 114 and structural layer 118are insulating materials. In some embodiments, structural layer 114 andstructural layer 118 are an oxide, nitride, or oxynitride. In a specificembodiment, structural layer 114 and structural layer 118 are siliconoxide. In another specific embodiment, structural layer 114 andstructural layer 118 are silicon nitride.

FIG. 2b shows MEMS gas sensor 111 including substrate 112, firstelectrode 116, second electrode 120, gas sensitive material 122, thirdelectrode 126, and gas sensitive material 132 overlying cavity 113. MEMSgas sensor 111 includes elements as described in reference to MEMS gassensor 110 with the addition of third electrode 126 and gas sensitivematerial 132. According to various embodiments, third electrode 126 isspaced apart from second electrode 120 by structural layer 124 andincludes perforations 138. In such embodiments, one or both ofenvironmental coupling 128 and environmental coupling 130 may beincluded in order to provide an environmental coupling between theambient environment and MEMS gas sensor 111 from the top, the bottom(through cavity 113), or both. Gas sensitive material 132 may be any ofthe materials described in reference to gas sensitive material 122,third electrode 126 may be any of the materials or structures describedin reference to first electrode 116 or second electrode 120, andstructural layer 124 may be any of the materials or structures describedin reference to structural layer 114 or structural layer 118.

In various embodiments, MEMS gas sensor 110 and MEMS gas sensor 111 maybe formed of layers compatible with a single or double backplatecapacitive microphone, respectively. In particular embodiments, thelayers of MEMS gas sensor 110 and MEMS gas sensor 111 are formed usingthe same processing steps and layer stack as a single or doublebackplate capacitive microphone, respectively, with the addition of gassensitive material 122 or gas sensitive material 132, as describedfurther herein below in reference to the other figures.

According to various embodiments, MEMS gas sensor 110 and MEMS gassensor 111 may include an integrated heating element that heats gassensitive material 122 or gas sensitive material 132. For example, whengas sensitive material 122 is sensitive to humidity and MEMS gas sensor110 operates as a humidity sensor, the integrated heating element mayheat up gas sensitive material 122 to remove the moisture absorbed ingas sensitive material 122 and reset MEMS gas sensor 110. In specificembodiments, any of first electrode 116, second electrode 120, and thirdelectrode 126 are configured as the integrated heating element. Inalternative embodiments, an additional heating element (not shown) maybe integrated on substrate 112 and thermally coupled to gas sensitivematerial 122 or gas sensitive material 132. In still more specificembodiments, the heating element may be in physical contact with gassensitive material 122 or gas sensitive material 132. In someembodiments, first electrode 116, second electrode 120, or thirdelectrode 126 may include patterned heating structures or elements (notshown) in the same device layer that are electrically insulated from therespective electrode.

In various embodiments, ventilation hole 115 passes through secondelectrode 120, structural layer 118, and first electrode 116 for MEMSgas sensor 110 or third electrode 126, structural layer 124, secondelectrode 120, structural layer 118, and first electrode 116 for MEMSgas sensor 111 and may serve as an access for both sides to the mediacoming in from either side through environmental coupling 128 orenvironmental coupling 130. In such embodiments, ventilation hole 115may be a single ventilation hole or multiple ventilation holes in thesensing and structural layers that provides a path around the sensingstructure for the media, e.g., gas, to contact gas sensitive material122 or gas sensitive material 132 from both directions while includingonly one of environmental coupling 128 and environmental coupling 130.In some embodiments, ventilation hole 115 may also pass through gassensitive material 122 or gas sensitive material 132. In still furtherembodiments, ventilation hole 115 may be formed in various layers orsubstrate 112 some distance from the gas sensor or cavity 113, but maystill provide a bypass ventilation path.

FIGS. 3a and 3b illustrate a top view and cross-sectional view of anembodiment integrated MEMS transducer 140 including gas sensor 160 andmicrophone 162. According to various embodiments, integrated MEMStransducer 140 is formed of top electrode layer 150 spaced apart frombottom electrode layer 146 by structural layer 148. Bottom electrodelayer 146, structural layer 148, and top electrode layer 150 are spacedfrom substrate 142 by structural layer 144 and are formed overlyingcavity 156 and cavity 158. In such embodiments, gas sensor 160 is acapacitive or resistive gas sensor as described hereinabove in referenceto MEMS gas sensor 110 in FIG. 2 a.

In various embodiments, gas sensor 160 includes gas sensitive material152 over cavity 156 and microphone 162 includes deflectable membrane 164and perforated backplate 166 over cavity 158. In such embodiments,deflectable membrane 164 and perforated backplate 166 are spaced apartby structural layer 148 and separated by an air gap over cavity 158.Deflectable membrane 164 is a part of, or formed by, top electrode layer150 and perforated backplate 166 is a part of, or formed by, bottomelectrode layer 146. Similarly, top electrode layer 150 forms the topelectrode for gas sensor 160 and bottom electrode layer 146 forms thebottom electrode for gas sensor 160.

In other embodiments, microphone 162 may alternatively be amicrospeaker. In some embodiments, microphone 162 may be implementedwith a second perforated backplate and gas sensor 160 may also beimplemented with a third electrode layer, as described in reference toMEMS gas sensor 111 in FIG. 2b . Although perforations are only shown inbottom electrode layer 146, perforations may be included in topelectrode layer 150, depending on an environmental coupling or port (notshown), as described hereinabove in reference to perforations 134 andperforations 136 in FIGS. 2a and 2b . In various embodiments, cavity 156and cavity 158 may be a same cavity or separate cavities in substrate142. In such embodiments, cavity 156 and cavity 158 may be formedseparately or simultaneously.

According to some embodiments, integrated MEMS transducer 140 furtherincludes temperature sensing element 161. In such embodiments,temperature sensing element 11 may be include in or on substrate 142, orany of the layers of integrated MEMS transducer 140. Temperature sensingelement 161 may be formed of any of the material described hereinabovein reference to first electrode 116 in FIG. 2a . In such embodiments,temperature sensing element 161 may be implemented as described inco-pending U.S. application Ser. No. 14/613,106, filed on Feb. 3, 2015,and entitled “System and Method for an Integrated Transducer andTemperature Sensor,” which is incorporated herein in its entirety.

According to various embodiments, integrated MEMS transducer 140 mayinclude an integrated heating element that heats gas sensitive material152 as similarly described hereinabove in reference to MEMS gas sensor110 and MEMS gas sensor 111 in FIGS. 2a and 2b . In such embodiments,the integrated heating element may be implemented using top electrodelayer 150 or bottom electrode layer 146 in the region where either layeris in contact with gas sensitive material 152. In other alternativeembodiments, a separate integrated heating structure (not shown) may beincluded on substrate 142 for heating gas sensitive material 152 assimilarly described hereinabove in reference to MEMS gas sensor 110 andMEMS gas sensor 111 in FIGS. 2a and 2 b.

According to various embodiments, the materials used for the structuresin FIGS. 3a and 3b are the same as described in reference tocorresponding structures in FIGS. 2a and 2b . Specifically, substrate142 may include any of the materials or structures described inreference to substrate 112, structural layer 144 may include any of thematerials or structures described in reference to structural layer 114,bottom electrode layer 146 may include any of the materials orstructures described in reference to first electrode 116, structurallayer 148 may include any of the materials or structures described inreference to structural layer 118, top electrode layer 150 may includeany of the materials or structures described in reference to secondelectrode 120, and gas sensitive material 152 may include any of thematerials or structures described in reference to gas sensitive material122. Further, description of perforations 134 and perforations 138 alsoapplies to perforations 154.

According to various embodiments, integrated MEMS transducer 140includes both gas sensor 160 and microphone 162 in substrate 142. Thus,both gas sensor 160 and microphone 162 may be formed simultaneouslyusing the same microfabrication sequence, with the addition of gassensitive material 152, and integrated on the same semiconductor die. Insome embodiments, temperature sensing element 161 is also integrated onthe same semiconductor die. Such embodiments may include components withincreased functionality and a very small increase in fabrication costand complexity.

FIGS. 4a and 4b illustrate a top view and cross-sectional view ofanother embodiment integrated MEMS transducer 170 including gas sensor172 and microphone 174. According to various embodiments, integratedMEMS transducer 170 is formed of top electrode layer 150 spaced apartfrom bottom electrode layer 146 by structural layer 148 as described inreference to integrated MEMS transducer 140 in FIGS. 3a and 3b . Gassensor 172 is a capacitive or resistive gas sensor as similarlydescribed hereinabove in reference to MEMS gas sensor 110 and gas sensor160 in FIG. 2a and FIGS. 3a and 3b ; with the exception that gas sensor172 is integrated in a central region of microphone 174. In someembodiments, integrated MEMS transducer 170 includes temperature sensingelement 161 as described hereinabove in reference to FIG. 3a .Microphone 174 may be microspeaker in some embodiments. Integrated MEMStransducer 170 may also include an integrated heating element asdescribed hereinabove in reference to integrated MEMS transducer 140 inFIGS. 3a and 3 b.

In such embodiments, gas sensitive material 152 is formed between topelectrode layer 150 and bottom electrode layer 146 in a central releasedregion overlying cavity 156. An air gap separates top electrode layer150 and bottom electrode layer 146 in a region surrounding gas sensitivematerial 152. The region with the air gap and without gas sensitivematerial 152 includes a deflectable membrane portion, formed from topelectrode layer 150, and a perforated backplate portion, formed frombottom electrode layer 146, which together operate as microphone 174.Thus, top electrode layer 150 and bottom electrode layer 146 form thesensing electrodes for both gas sensor 172 and microphone 174. In suchembodiments, deflections of top electrode layer 150, as the deflectablemembrane for microphone 174, occur in the portion surrounding gassensitive material 152, while the region immediately above gas sensitivematerial 152 is fixed to gas sensitive material 152 and does notsubstantially deflect.

According to various embodiments, both top electrode layer 150 andbottom electrode layer 146 are segmented into two separate electroderegions in order to provide electrodes for both microphone 174 and gassensor 172 in the same layer. Segmentation regions 176 providesegmentation of both top electrode layer 150 and bottom electrode layer146. In various embodiments, segmentation regions 176 may be formed ofan insulating material. In specific embodiments, segmentation regions176 are formed of an oxide, nitride, or oxynitride. In otherembodiments, segmentation regions 176 are formed only by removingportions of top electrode layer 150 and bottom electrode layer 146 insegmentation regions 176. In some alternative embodiments, segmentationregions 176 pass completely through gas sensitive material 152 and mayform a continuous layer.

Further, electrical connections to the two separate electrode regions ofboth top electrode layer 150 and bottom electrode layer 146 are providedby metallization or doped semiconductors, for example (not shown). Suchelectrical connections generally include additional segmentation orinsulation (not shown) formed on or in top electrode layer 150 andbottom electrode layer 146, as will be readily understood by one havingordinary skill in the art.

FIGS. 5a and 5b illustrate a top view and cross-sectional view of afurther embodiment integrated MEMS transducer 180 including gas sensor182 and microphone 184. According to various embodiments, integratedMEMS transducer 180 is similar to integrated MEMS transducer 170described hereinabove in reference to FIGS. 4a and 4b ; with theexception that gas sensor 182 is integrated in a peripheral regionsurrounding microphone 184.

In such embodiments, gas sensitive material 152 is formed in theperipheral region adjacent to structural layer 148 between top electrodelayer 150 and bottom electrode layer 146 and above cavity 156. Thecentral portion of top electrode layer 150 forms the deflectablemembrane for microphone 184 and the central portion of bottom electrodelayer 146 forms the perforated backplate for microphone 184. Similarly,the peripheral region of top electrode layer 150 and the peripheralregion of bottom electrode layer 146 together form the top and bottomelectrodes, respectively, for gas sensor 182. As described in referenceto FIG. 4b , segmentation regions 176 in top electrode layer 150 andbottom electrode layer 146 separate the sensing electrodes for gassensor 182 from the sensing electrodes, membrane and backplate, formicrophone 184. In some embodiments, integrated MEMS transducer 180includes temperature sensing element 161 as described hereinabove inreference to FIG. 3a . Microphone 184 may be microspeaker in someembodiments. Integrated MEMS transducer 180 may also include anintegrated heating element as described hereinabove in reference tointegrated MEMS transducer 140 in FIGS. 3a and 3 b.

FIGS. 6a and 6b illustrate a top view and cross-sectional view ofanother embodiment MEMS gas sensor 190 including first interdigitatedelectrode 192 and second interdigitated electrode 194. According tovarious embodiments, first interdigitated electrode 192, secondinterdigitated electrode 194, and membrane layer 196 together form astructured membrane that has interdigitated electrodes filled with gassensitive material 152 in the gaps between first interdigitatedelectrode 192 and second interdigitated electrode 194. As similarlydescribed hereinabove, gas sensitive material 152 may be implemented asa material with a resistance value or dielectric constant that dependson a specific gas concentration. In such embodiments, firstinterdigitated electrode 192 and second interdigitated electrode 194 mayfunction as electrodes for a resistive gas sensor or a capacitive gassensor. Further description of interdigitated electrodes is included inco-pending U.S. application Ser. No. 13/743,306, filed Jan. 16, 2013,and entitled “Comb MEMS Device and Method of Making a Comb MEMS Device,”which is incorporated herein by reference in its entirety, and inco-pending U.S. application Ser. No. 13/947,823, filed Jul. 22, 2013,and entitled “MEMS Device,” which is incorporated herein by reference inits entirety. Various embodiments described herein may be combined withfabrication techniques for a comb drive or interdigitated electrode MEMSsensor in order to fabricate MEMS gas sensor 190. MEMS gas sensor 190may also include an integrated heating element as described hereinabovein reference to integrated MEMS transducer 140 in FIGS. 3a and 3b orMEMS gas sensor 110 in FIG. 2a , for example.

In various embodiments, membrane layer 196 is spaced from substrate 142by structural layer 144. First interdigitated electrode 192 and secondinterdigitated electrode 194 may be formed of conductive materials, suchas described hereinabove in reference to first electrode 116 and secondelectrode 120 in FIG. 2a . Further, first interdigitated electrode 192and second interdigitated electrode 194 are formed as ridges or finsthat extend through membrane layer 196 and gas sensitive material 152.Membrane layer 196 may be formed of an insulating material, such as anoxide, nitride, or oxynitride. In other embodiments, membrane layer 196is formed of the same material as first interdigitated electrode 192 andsecond interdigitated electrode 194 and additional segmentation ofinsulation (not shown) is included between first interdigitatedelectrode 192 and second interdigitated electrode 194.

FIG. 7 illustrates a block diagram of an embodiment fabrication process200 for forming a MEMS transducer, where fabrication process 200includes steps 202-222. According to various embodiments, fabricationprocess 200 begins with a substrate in step 202. The substrate may beformed of a semiconductor, such as silicon, or as another material, suchas a polymer for example. An etch stop layer is formed on the substratein step 204. The etch stop layer may be silicon oxide formed usingtetraethyl orthosilicate (TEOS), for example. In other embodiments, theetch stop layer may be silicon nitride.

In step 206, a first electrode layer is formed by forming and patterninglayers for the first electrode layer. Step 206 may include depositing asingle layer for the first electrode layer or multiple layers for thefirst electrode layer. As an example of multiple layers, step 206 mayinclude depositing an insulating layer, such as SiN, depositing aconducting layer, such as polysilicon, patterning the conducting layer,depositing another insulating layer, such as SiN, and patterning theresulting stack of layers. Patterning may include a photolithographicprocess to produce the first electrode layer structure withperforations. As an example of a single layer, step 206 may includedepositing a conducting layer, such as polysilicon, and patterning theconducting layer. In various embodiments, the first electrode layer maybe any of the materials described hereinabove in reference to firstelectrode 116 in FIG. 2a . In some embodiments, step 206 also includesforming a temperature sensing element in or on the substrate or thefirst electrode layer, such as temperature sensing element 161 describedhereinabove in reference to FIGS. 3a and 3 b.

In other embodiments, structure variations and material alternatives areenvisioned. In some alternative embodiments, the first electrode layermay be formed of any number of layers, conductive or insulating. Forexample, in some embodiments, the first electrode layer may includelayers of metals, semiconductors, or dielectrics. In some embodiments,the first electrode layer may be formed of silicon on insulator (SOI) ormetal and dielectric layers.

In various embodiments, step 208 includes forming and patterning astructural material, such as TEOS, and a gas sensitive material. Formingand patterning in step 208 is performed in order to provide spacing fora second electrode layer in a region for a MEMS microphone ormicrospeaker and to provide the gas sensitive material in a region for aMEMS gas sensor. In various embodiments, the first electrode layer andthe second electrode layer may be part of a backplate and a membrane forthe MEMS microphone or microspeaker and also part of a first electrodeand a second electrode for the MEMS gas sensor, as described hereinabovein reference to the other figures. According to various embodiments, thegas sensitive material of step 208 may be patterned to form any of thegas sensors as described hereinabove in reference to the other figures.Further, the gas sensitive material of step 208 may include any of thematerials described in reference to gas sensitive material 122 in FIG.2a . The structural layer of step 208 may be patterned in order to formanti-stiction bumps for the second electrode layer. In addition, thestructural layer formed in step 208 may include multiple depositions anda planarization step, such as a chemical mechanical polish (CMP).

Step 210 includes forming the second electrode layer and patterning thesecond electrode layer. The second electrode layer may be formed ofpolysilicon, for example. In other embodiments, the second electrodelayer may be formed of other conductive materials, such as a dopedsemiconductor or a metal, for example. In various embodiments, thesecond electrode layer may be any of the materials described inreference to second electrode 120 in FIG. 2a . Patterning the secondelectrode layer in step 210 may include a photolithographic process, forexample, that defines the shapes or structures of second electrodes forboth a MEMS gas sensor and a MEMS microphone. The second electrode layermay include anti-stiction bumps in a region for the membrane of themicrophone based on the structure formed in step 208.

In some embodiments, the step of forming the second electrode layer instep 210 may also include a step of forming a temperature sensingelement formed in or on the same layer as the second electrode layer.According to various embodiments, only a single backplate microphone isformed and, therefore, only the first electrode layer and the secondelectrode layer are formed. In such embodiments, step 212 and step 214may be omitted.

In various embodiments for forming a dual backplate microphone, step 212includes forming and patterning additional structural material, such asTEOS, and a gas sensitive material. Similar to step 208, the structuralmaterial may be formed and patterned in step 212 to space a thirdelectrode layer, which may form a second backplate, from the secondelectrode layer in a region for the MEMS microphone or microspeaker andthe gas sensitive material may be formed and patterned in a region forthe MEMS gas sensor. Step 212 may include depositing any of thematerials described in reference to step 208. Step 214 includes formingand patterning the layers of the third electrode layer. In someembodiments, forming and patterning in step 214 includes deposition oflayers and photolithographic patterning, for example. In variousembodiments, step 214 may include similar features and materials asdescribed hereinabove in reference to the first electrode layer formedin step 206 and the same description applies. Thus, in variousembodiments, step 214 may also include forming a temperature sensingelement.

Following step 214, step 216 includes forming and patterning additionalstructural material in various embodiments. The structural material maybe a TEOS oxide. In some embodiments, the structural material isdeposited as a sacrificial material or a masking material for subsequentetch or patterning steps. Step 218 includes forming and patterningcontact pads. Forming and patterning the contact pads in step 218 mayinclude etching contact holes in the existing layers to provide openingsto the third electrode layer, the second electrode layer, firstelectrode layer, the substrate, and the temperature sensing element. Ina specific embodiment, the temperature sensing element may have twoopenings for two contact pads that couple to the temperature sensingelement.

After forming the openings to each respective structure or layer, thecontact pads may be formed by depositing a conductive material, such asa metal, in the openings and patterning the conductive material to formseparate contact pads. Step 218 may also include forming a temperaturesensing element in metallization layers or between contact pads. Forexample, a resistive element with a resistance dependent on temperaturemay be formed between additional contact pads. In one specificembodiment, a platinum wire is formed between contact pads in step 218.In a still further embodiment, step 218 may also include forming a backcontact for measuring a spreading resistance of the substrate.

In various embodiments, step 220 includes performing a backside etch,such as a Bosch etch. The backside etch forms a cavity in the substratein order to form a sound port to the fabricated microphone andtemperature sensor or to form a reference cavity. Step 222 includesperforming a release etch to remove the structural materials protectingand securing the first electrode layer, the second electrode layer, andthe third electrode layer. Following the release etch in step 222, themembrane, formed by a portion of the second electrode layer for example,of the microphone may be free to move in some embodiments.

Fabrication process 200 may be modified in specific embodiments toinclude only a single backplate and membrane for the integratedmicrophone. Those of skill in the art will readily appreciate thatnumerous modifications may be made to the general fabrication sequencedescribed hereinabove in order to provide various benefits andmodifications known to those of skill in the art while still includingvarious embodiments of the present invention. In some embodiments,fabrication sequence 200 may be implemented to form a microspeakerhaving a similar structure as a single backplate MEMS microphone, forexample. In other embodiments, fabrication sequence 200 may beimplemented to form only a gas sensor and no integrated microphone. Invarious embodiments, fabrication sequence 200 may also include formingseparate heating elements in contact with the gas sensitive material.

FIG. 8 illustrates a block diagram of another embodiment method offorming 250 a MEMS transducer, where method of forming 250 includessteps 252-260. According to various embodiments, step 252 includesforming a first electrode plate on a substrate. Step 254 includesforming a second electrode plate spaced apart from the first electrodeplate. Step 256 includes exposing a bottom surface of the firstelectrode plate by etching a cavity in a backside of the substrate. Thecavity may be connected to a port in a package for the MEMS transducer,where the port provides an opening, or coupling, to the ambientenvironment outside of and surrounding the package.

In various embodiments, step 258 includes releasing the first electrodeplate and the second electrode plate. After release, the first electrodeplate and the second electrode plate are released from below, in thecavity, and above. In such embodiments, the first electrode plate andthe second electrode plate may still be connected by a gas sensitivematerial formed (in step 260) between the first and second electrodeplates, for example, as described hereinabove in reference to FIGS. 2a,2b, 3a, 3b, 4a, 4b, 5a, 5b, 6a, and 6b , for example.

According to various embodiments, step 260 includes forming a gassensitive material on the first electrode plate. In some embodiments,the gas sensitive material is a gas sensitive dielectric material thathas a dielectric constant that varies in relation to the concentrationof a specific gas. For example, the gas sensitive dielectric materialhas a dielectric constant that varies with humidity, i.e., theconcentration of water vapor in the air. The gas sensitive dielectricmaterial may instead be a resistive material as described hereinabove inreference to the other figures.

In further embodiments, method of forming 250 may include additionalsteps in different orders. For example, method of forming 250 mayinclude additional steps for forming a third electrode plate. In someembodiments, a microphone is formed during the same steps 252-260 forthe MEMS transducer. Thus, a gas sensor and a microphone are formedsimultaneously using the same electrode layers. In still furtherembodiments, a temperature sensing element is also formed during methodof forming 250.

According to various embodiments, step 260 may be performed at the endof method of forming 250 as a post-processing step. In such embodiments,the gas sensitive material, as a dielectric or a resistive material, isdispensed, printed, or applied to the released MEMS system. In someembodiments, the gas sensitive material may be applied only to sensingareas, e.g., not to a microphone portion, as described hereinabove inreference to FIGS. 2 and 3. In other embodiments, the gas sensitivematerial may be applied to a capacitive plate sensor within specific gassensor portions separated from acoustic transducer portions as describedhereinabove in reference to FIGS. 4 and 5. In still other embodiments,the gas sensitive material may be applied to interdigitated electrodesas described hereinabove in reference to FIG. 6. In such embodiments,the post-processing filling with gas sensitive material may be appliedsuch that capillary forces fill the gaps between interdigitatedelectrodes with gas sensitive material and the applied amount of gassensitive material limits the filling volume between the gaps. In otherembodiments, step 260 is performed between step 252 and step 254 andincludes depositing the gas sensitive material on top of the firstelectrode plate before forming the second electrode plate.

According to an embodiment, a microelectromechanical systems (MEMS)transducer includes a substrate with a first cavity that passes throughthe substrate from a backside of the substrate. The MEMS transducer alsoincludes a perforated first electrode plate overlying the first cavityon a topside of the substrate, a second electrode plate overlying thefirst cavity on the topside of the substrate and spaced apart from theperforated first electrode plate by a spacing region, and a gassensitive material in the spacing region between the perforated firstelectrode plate and the second electrode plate. The gas sensitivematerial has an electrical property that is dependent on a concentrationof a target gas. Other embodiments include corresponding systems andapparatus, each configured to perform corresponding embodiment methods.

Implementations may include one or more of the following features. Invarious embodiments, the MEMS transducer further includes a MEMSacoustic transducer integrated on the substrate, where the MEMS acoustictransducer includes a perforated backplate overlying a second cavity inthe substrate, and a deflectable membrane overlying the second cavityand spaced apart from the perforated backplate. In some embodiments, theperforated backplate is formed of a same semiconductor layer as theperforated first electrode plate, and the deflectable membrane is formedof a same semiconductor layer as the second electrode plate. The firstcavity and the second cavity may be a same cavity that passes throughthe substrate from a backside of the substrate.

In various embodiments, the perforated first electrode plate includes acentral portion of the perforated backplate and the second electrodeplate includes a central portion of the deflectable membrane. In otherembodiments, the perforated first electrode plate includes a peripheralportion of the perforated backplate and the second electrode plateincludes a peripheral portion of the deflectable membrane.

In various embodiments, the target gas is water vapor and the gassensitive material is a humidity sensitive material. In specificembodiments, the humidity sensitive material is polyimide. In otherembodiments, the target gas is carbon dioxide.

In various embodiments, the MEMS transducer further includes aperforated third electrode plate overlying the first cavity on thetopside of the substrate and spaced apart from the second electrodeplate by an additional spacing region, and also further includes the gassensitive material in the additional spacing region between theperforated third electrode plate and the second electrode plate. Theperforated first electrode plate may be coupled to an ambientenvironment through the first cavity and a port in a package containingthe MEMS transducer. In some embodiments, the perforated first electrodeplate overlies the second electrode plate and is coupled to an ambientenvironment through a top port in a package containing the MEMStransducer.

In various embodiments, the MEMS transducer further includes atemperature sensing element integrated on the substrate. The MEMStransducer may further include a heating element in physical contactwith the gas sensitive material. In some embodiments, the MEMStransducer further includes a ventilation opening that bypasses theperforated first electrode plate and the second electrode plate andprovides a ventilation path between the first cavity and the topside ofthe substrate. According to various embodiments, the gas sensitivematerial includes a gas sensitive dielectric material that has adielectric constant that is dependent on the concentration of the targetgas.

According to an embodiment, a method of fabricating a MEMS sensorincludes forming a first electrode plate on a substrate, forming asecond electrode plate spaced apart from the first electrode plate,exposing a bottom surface of the first electrode plate by etching acavity in a backside of the substrate, releasing the first electrodeplate and the second electrode plate, and forming a gas sensitivematerial between the first electrode plate and the second electrodeplate. Other embodiments include corresponding systems and apparatus,each configured to perform corresponding embodiment methods.

Implementations may include one or more of the following features. Invarious embodiments, forming the gas sensitive material includesdispensing the gas sensitive material as a liquid into a region betweenthe first electrode plate and the second electrode plate after releasingthe first electrode plate and the second electrode plate. Dispensing thegas sensitive material as a liquid may include dispensing polyimide as aliquid. In other embodiments, forming the gas sensitive materialincludes depositing the gas sensitive material on the first electrodeplate before forming the second electrode plate.

In various embodiments, the gas sensitive material includes a gassensitive dielectric material with a dielectric constant that isproportional to a concentration of a target gas. In some embodiments,forming the first electrode plate includes forming a perforated firstelectrode plate. Forming the perforated first electrode plate on thesubstrate may also include forming a perforated backplate, forming thegas sensitive material may include forming the gas sensitive material ina first region over the perforated backplate and forming an air gap in asecond region over the perforated backplate, and forming the secondelectrode plate on the gas sensitive material may further includeforming a deflectable membrane over the air gap in the second region. Insome embodiments, the first region is a peripheral region and the secondregion is a central region. In other embodiments, the second region is aperipheral region and the first region is a central region.

In various embodiments, the method further includes forming a perforatedbackplate simultaneous to forming the perforated first electrode plate,where the perforated backplate and the perforated first electrode plateare formed of a same semiconductor layer. In such embodiments, themethod also further includes forming a deflectable membrane simultaneousto forming the second electrode plate, where the deflectable membraneand the second electrode plate are formed of a same semiconductor layerand the deflectable membrane is spaced apart from the perforatedbackplate. In some embodiments, the method further includes forming aheating element thermally coupled to the gas sensitive material. Themethod may also further include forming a temperature sensing element onthe substrate.

According to an embodiment, a MEMS transducer includes a MEMS acoustictransducer that includes a perforated backplate overlying a substrateand a membrane overlying the substrate and spaced apart from theperforated backplate by a first spacing. The MEMS transducer furtherincludes a MEMS gas sensor that includes a perforated first electrodeoverlying the substrate, a second electrode overlying the substrate andspaced apart from the perforated first electrode by a second spacing,and a gas sensitive dielectric between and in contact with theperforated first electrode and the second electrode. Other embodimentsinclude corresponding systems and apparatus, each configured to performcorresponding embodiment methods.

Implementations may include one or more of the following features. Invarious embodiments, the perforated backplate and the perforated firstelectrode are formed of a same semiconductor layer. The substrate mayinclude a cavity, and the perforated first electrode, the secondelectrode, and the gas sensitive dielectric overly the cavity. In someembodiments, the MEMS transducer further includes a temperature sensingelement integrated on the substrate. In further embodiments, the MEMStransducer also includes a heating element in physical contact with thegas sensitive dielectric.

In various embodiments, the perforated first electrode is formed in acentral region of the perforated backplate and the second electrode isformed in a central region of the membrane. In other embodiments, theperforated first electrode is formed in a peripheral region of theperforated backplate and the second electrode is formed in a peripheralregion of the membrane.

According to some embodiments described herein, advantages may includeadding functionality to an integrated product. Some embodiments mayinclude an acoustic transducer and a gas sensor integrated in a samesemiconductor die with shared functional device layers, such aselectrode layers. Such embodiments may advantageously include no extramasks or only a single extra mask during the fabrication sequence whencompared to an equivalent microphone fabrication sequence. Anotheradvantage of some embodiments may include integrated temperature sensingfunctionality. Integration in a single semiconductor may alsoadvantageously lead to improved performance and decreased cost in someembodiments. A further advantage of some embodiments may includeproviding environmental coupling for a group of sensors through a singlehole or port, or shared holes or ports, in a single package. Suchembodiments may advantageously improve performance, increaseinterchangeability of sensor systems, and simplify design constraints.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A method of fabricating a microelectromechanicalsystems (MEMS) sensor, the method comprising: forming a first electrodeplate on a substrate, wherein the first electrode plate comprises aperforated backplate and a perforated first electrode plate; forming asecond electrode plate spaced apart from the first electrode plate,wherein the second electrode plate comprises a deflectable membrane;exposing a bottom surface of the first electrode plate by etching acavity in a backside of the substrate; releasing the first electrodeplate and the second electrode plate; and forming a gas sensitivematerial between the first electrode plate and the second electrodeplate, wherein forming the gas sensitive material comprises forming thegas sensitive material in a first region over the perforated backplateand forming an air gap in a second region over the perforated backplate,and wherein forming the second electrode plate comprises forming thedeflectable membrane over the air gap in the second region.
 2. Themethod of claim 1, wherein forming the gas sensitive material comprisesdispensing the gas sensitive material as a liquid into a region betweenthe first electrode plate and the second electrode plate after releasingthe first electrode plate and the second electrode plate.
 3. The methodof claim 2, wherein dispensing the gas sensitive material as a liquidcomprises dispensing polyimide as a liquid.
 4. The method of claim 1,wherein forming the gas sensitive material comprises depositing the gassensitive material on the first electrode plate before forming thesecond electrode plate.
 5. The method of claim 1, wherein the gassensitive material comprises a gas sensitive dielectric material with adielectric constant that is proportional to a concentration of a targetgas.
 6. The method of claim 1, wherein the first region is a peripheralregion and the second region is a central region.
 7. The method of claim1, wherein the second region is a peripheral region and the first regionis a central region.
 8. The method of claim 1, further comprising:forming the perforated backplate simultaneous to forming the perforatedfirst electrode plate, wherein the perforated backplate and theperforated first electrode plate are formed of a same semiconductorlayer; and forming the deflectable membrane simultaneous to forming thesecond electrode plate, wherein the deflectable membrane and the secondelectrode plate are formed of a same semiconductor layer, and whereinthe deflectable membrane is spaced apart from the perforated backplate.9. The method of claim 1, further comprising forming a heating elementthermally coupled to the gas sensitive material.
 10. The method of claim1, further comprising forming a temperature sensing element on thesubstrate.
 11. A method fabricating a microelectromechanical systems(MEMS) sensor, the method comprising: depositing a first electrode layerover a topside of a substrate; patterning the first electrode layer toform a first electrode plate; forming a gas-sensitive material over thefirst electrode plate, wherein the gas-sensitive material has anelectrical property that is dependent on a concentration of a targetgas; depositing a second electrode layer over the gas-sensitivematerial; patterning the second electrode layer to form a secondelectrode plate; etching the substrate from a backside of the substrateto form a first cavity, the first cavity exposing a surface of the firstelectrode layer directed away from the gas-sensitive material; etchingthe substrate from the backside of the substrate to form a secondcavity; and integrating a MEMS acoustic transducer on the substrate, theMEMS acoustic transducer comprising a perforated backplate overlying thesecond cavity, and a deflectable membrane overlying the second cavityand spaced apart from the perforated backplate.
 12. The method of claim11, wherein patterning the first electrode layer comprises aphotolithographic process.
 13. The method of claim 11, wherein the firstelectrode layer comprises polysilicon.
 14. The method of claim 11,wherein etching the substrate from the backside of the substrate to formthe first cavity is simultaneous to etching the substrate from thebackside of the substrate to form the second cavity.
 15. The method ofclaim 11, wherein integrating the MEMS acoustic transducer on thesubstrate comprises: patterning the first electrode layer to form theperforated backplate; and patterning the second electrode layer to formthe deflectable membrane.
 16. The method of claim 11, wherein the targetgas is water vapor and the gas-sensitive material is a humiditysensitive material.
 17. The method of claim 16, wherein the humiditysensitive material comprises a polyimide.
 18. The method of claim 5,wherein the target gas comprises at least one of hydrogen, carbondioxide, oxygen, ethylene, or ammonia.
 19. The method of claim 18,wherein the target gas comprises hydrogen, and wherein the gas sensitivematerial comprises a ferroelectric material.
 20. The method of claim 18,wherein the target gas comprises carbon dioxide, and wherein the gassensitive material comprises a fluoropolymer material.